The present invention relates to filter elements with changing devices and contamination protection located in the waste tanks, for separating substances which are harmful to the health from the breathing or process air of ventilation systems used in the nuclear field.
According to the present invention, as required, filter layers made from filter paper or granular loose material in the form of sheets, blocks, parallelipipeds, disks, rings, star-shaped member or zig-zag-shaped members are placed in standard 200 or 400 liter waste tanks without any external modifications.
Furthermore, each of the waste tanks is provided with a detachable cover, as well as with a supply air and spent air pipe for the direct connection to superimposed dust laden and clean air ducts.
Transparent protective hoses can be fixed to multiple edge rings arranged on the cover and waste tanks for contamination-free operation and for replacing the used tank filters.
According to the present invention, the covers of the waste tank filters or drum filters are provided with remote control components which serve, after closing the flaps, to either detach the waste tank with the fitted filter from the seals or to fix a new filter tank to the air carrying ducts.
Other filtering problems may occur which may require putting out of commission and the dismantling of no longer usable nuclear piles. An easy and safe procedure for changing filters containing nuclear waste is needed.
Uneconomical operation, expired service life or the no longer correctable consequences of a fault are among the reasons for demolition, which lead to such unusual decisions.
It is only in recent times that acceptance requirements have been placed on nuclear power stations in connection with the design of such stations which are easy to maintain, repair and shut down.
Serious environmental protection problems can occur when dismantling and demolishing radioactive ruins and restoring the area to its original state. The shutting down of a reactor does not imply the end of radioactivity and in fact for many years the radio activity is stored deep in the materials of the hot nuclear area. To give but a few examples, these include the core casing of the containment vessel, as well as the various fittings, pipes, pumps and steam generators on which radioactive deposits have collected. Even the concrete envelope of the biological shield area is partly affected by radioactive deposits.
The complete demolition of a nuclear reactor firstly implies that all contaminated parts made from metal or concrete must be crushed under contamination-proof conditions before being transporting away to the final storage point. However, no difficulties are involved in the dismantling of so-called cold power station remains. Demolition methods, used on a trial basis in the U.S., have only in part provided solutions in connection with how such a large scale demolition project can be carried out.
Demolition methods using plasma torch cutting machines in cabins with a high number of air changes or with portable free-burning stations offer better chances for dry demolition than for wet demolition. The problem of disposing of the contaminated liquid would only make the latter process more difficult.
Finely dispersed melting smoke of high density and concentration is produced when cutting up reactor parts, particularly when such parts are made from high-grade steel. This also applies during the flame cutting of reinforced concrete or steel parts. Large amounts of dust from burnt concrete mix with the melting smoke due to oxidation of the metals, so that the dust concentrations in the air can be up to 1 to 2 g/m.sup.3 with particle sizes of 0.03 to 30 micrometers. During burning further difficulties occur due to flying sparks or smouldering residues, linked with increased air temperatures, which are more particularly a problem when aluminium oxide particles are contained in the smoke. Furthermore, e.g. a central accumulation of more than 400 g of dust from the highly active area is not possible, because the radiation dose of more than 10 mrem at a distance of 1 meter must not be exceeded for safety reasons.
From the filtering standpoint the problem is to remove harmful emission substances from the waste air in order to reduce the environmental hazards to a minimum. The degree of environmental pollution necessarily drops with the reduction in the dust elimination paths from the separation or cutting station to the waste tanks and from there to the long-term final storage location.
The hitherto proposed filter constructions for the indicated case are too costly from the apparatus standpoint and are therefore very cost-intensive. The numerous intermediate stations from the dust formation point to the dust collection containers with the long distances up to the point of filling into the standard waste tanks are subject to a high risk due to the spreading hazards and are only suitable if account can be taken of the aforementioned difficulties, such as e.g. occur when using electrostatic precipitators, tube filters or top-quality cyclone combinations with series-connected fine filters.